A standard stepping transformer has a pair of contact arms that are sequentially stepped angularly along circular arrays of contacts. The arms are moved one after the other for proper make/break sequencing. This is done as described, for instance, in German patent 1,220,033 of E. Baumgartner by a dual geneva escapement. One of the arms is carried on one end of a core shaft whose other end carries an upper geneva wheel and the other arm is carried on a tube shaft surrounding this core shaft and carrying on its other end a lower geneva wheel spaced beneath the upper wheel, although of course inverted or side-by-side orientation is also possible. Between these wheels is a driver disk having on its upper face an upper abutment engageable in the radially outwardly open slots of the upper wheel and on its lower face a lower abutment engageable in the similar slots of the lower wheel. The two abutments are spaced apart by 1808.degree. and the driver disk is moved periodically through this angle to step the contact arms angularly, one being moved with each half rotation of the driver disk.
It is extremely important to impede the contact arms from moving full circle. Thus their angular movement must be within less than 360.degree.. Overtravel would without fail result in damage to the switching arrangement.
Accordingly, German patent 1,039,129 of E. Bottger proposes a mechanical end-position latch that which has a separate blocking disk that is coaxial with the selector and that coacts with a pawl that can also be actuated by the selector. Such a separate mechanism is complex and expensive, and is not easy to adapt to blocking movement of two independently movable coaxial wheels.
German patent document 2,608,051 describes a system wherein the actuation of a coarse selector is effected by the geneva wheel which drives the fine selector.
In German 2,339,973 the geneva wheel carries an abutment which at an appropriate time engages a switchover or selector element. In this arrangement it is advantageous that these parts that are used to actuate the reverser or preselector employ elements such as levers, geneva segments and the like as described in German 3,017,790. Although this arrangement is relatively simple it does not impede all types of overtravel of the parts. In particular it does not positively block overtravel of both geneva wheels.
In fact none of the above-discussed systems allows two concentric geneva or maltese-cross wheels to be solidly locked against further movement in two end positions. It is possible to block the upper wheel, but if something goes wrong it is still possible for the driver to continue rotating and for the lower wheel to overtravel, potentially burning out the stepping transformer. Only by expensive and complex duplication of the upper-wheel blocking system is it possible to have a failsafe system, and such extra structure adds excessively to the cost of the arrangement.